This invention relates to electronic watthour meters, and more particularly to an electronic watthour meter in which its light load characteristic is improved and its characteristic with time is stablized.
Since an electronic watthour meter has no mechanically movable components, it is excellent in characteristic with time; that is, its accuracy is maintained unchanged for a long period of time. Furthermore, it is advantageous in that its size is relatively small, it is suitable for mass production and the manufacturing cost can be reduced. The watthour meter has a feature that change of a charging rate, as to an integrated power value, or its remote control can be realized by a simple electrical circuit.
Accordingly, it seems that electronic watthour meters will replace induction type watthour meters which are essentially formed with mechanical components. A variety of electronic watthour meters have been proposed in the art.
These conventional electronic watthour meters have an equivalent circuit as shown in FIG. 1. Thus, the watthour meter comprises: a multiplication circuit M in which a voltage signal e.sub.v, proportional to the load voltage of the power supply line, and a voltage signal e.sub.i, proportional to the consumption current of the supply line, are subjected to multiplication thereby to provide a voltage signal (e.sub.o =K.multidot.e.sub.v .multidot.e.sub.i where K is the constant) proportional to the instantaneous power of the supply line; and a voltage-to-frequency conversion circuit VF in which the output voltage signal e.sub.o, of the multiplication circuit M, is subjected to integration to provide a frequency signal f.sub.out. Accordingly, the watthour meter obtains an integrated power value by counting the frequency signal f.sub.out outputted by the voltage-to-frequency conversion circuit VF.
However, it should be noted that the multiplication circuit M and the voltage-to-frequency conversion circuit VF, of the electronic watthour meter, are made up of operational amplifiers, and therefore an offset voltage is certainly developed, with the result that it affects the accuracy of measurement of the watthour meter, because the accuracy of the watthour meter is expressed by an absolute error with respect to a true measurement value instead of a relative error with respect to the full scale (rating). Accordingly, the accuracy of the watthour meter should be so guaranteed as to be within the absolute error even if the input is 1/30 (3.33%) of the 100% rating or 1/50 (2%). If it is assumed that the rating of the voltage signal e.sub.i, corresponding to the consumption current, is 5 V for instance, then the error of 0.5% corresponds to an input 25 mV in conversion value, with the rating being 100%. Therefore, if the input in conversion value is of the order of 25 mV, then the accuracy will not be so seriously affected. However, in the case of 0.5% error (light load) with respect to a 1/50 input, the input in conversion value is 0.5 mV. Accordingly, it is necessary to reduce to less than 0.5 mV the offset voltages induced by the operational amplifiers in the multiplication circuit M and the voltage-to-frequency conversion circuit VF. However, it is considerably difficult to eliminate the offset voltage from the operational amplifier. In addition, the offset voltage is varied with time and temperature. Accordingly, it is difficult to maintain the electronic watthour meter with high accuracy.